Additive fabrication processes for producing three dimensional articles are known in the field. Additive fabrication processes utilize computer-aided design (CAD) data of an object to build three-dimensional parts layer-by-layer. These three-dimensional parts may be formed from liquid resins, powders, or other materials.
A non-limiting example of an additive fabrication process is stereolithography (SL), Stereolithography is a well-known process for rapidly producing models, prototypes, patterns, and production parts in certain applications. SL uses CAD data of an object wherein the data is transformed into thin cross-sections of a three-dimensional object. The data is loaded into a computer which controls a laser beam that traces the pattern of a cross section through a liquid radiation curable resin composition contained in a vat, solidifying a thin layer of the resin corresponding to the cross section. The solidified layer is recoated with resin and the laser beam traces another cross section to harden another layer of resin on top of the previous layer. The process is repeated layer by layer until the three-dimensional object is completed. When initially formed, the three-dimensional object is, in general, not fully cured and therefore may be subjected to post-curing, if required. An example of an SL process is described in U.S. Pat. No. 4,575,330.
The most used method of creating parts from a liquid radiation curable resin is stereolithography. Stereolithography is a vat-based additive fabrication process. Vat-based systems consist of a large reservoir, or vat, of liquid radiation curable resin wherein the imaging occurs. A vertically movable elevator platform is submersed in the vat and is used for supporting the solid three-dimensional object as it is built. Additionally, a recoater is present to assist in forming the next layer of liquid radiation curable resin. The recoater is present at the surface of the liquid radiation curable resin and movable across this surface to assist in forming the next layer of liquid radiation curable resin.
Typically, the building process in vat-based systems is a recurring process consisting of the following repeating steps: 1) the surface of the liquid radiation curable resin is exposed to appropriate imaging radiation by the tracing of a desired cross-section of the three-dimensional object by a laser, thus forming a solid layer; 2) the vertically movable elevator is translated down, further below the surface of the liquid radiation curable resin; 3) the recoater is translated across the surface of the liquid radiation curable resin to assist in forming the next layer of liquid radiation curable resin; and 4) the elevator is translated up such that the distance between the surface of the liquid radiation curable resin and the just formed solid layer of the three-dimensional object is equal to the desired thickness of the layer to be formed. Optionally, there may be a programmed dwell time before the steps are repeated in order to allow the liquid radiation curable resin to equilibrate such that a uniform layer thickness is ensured.
Generally, there are two elements of a stereolithography process that inhibit process speed: 1) the tracing of the laser beam, and 2) the recoating process and optional dwell time. Recently, new additive fabrication systems have been developed with the aim of improving the layer-by-layer process speed.
Firstly, the tracing of the laser beam in a stereolithography process inhibits build speed. The speed of the tracing step is highly dependent on the area and complexity of the cross-section. More tracing must occur for a larger, more complex cross-section than for a smaller, relatively simple one.
Additive fabrication systems have been developed which do not use lasers as the source of imaging radiation in order to make the imaging step less dependent on the complexity of the cross-section. Primarily, the new imaging source is a projection from a DMD (Digital Micromirror Device) or a LCD (Liquid Crystal Display) projector. DMD-based systems utilize a special chip comprising thousands of microscopic mirrors that correspond to pixels of the image. When using such a system in an additive fabrication process, an imaging time that is independent of cross-section complexity can be achieved. Please see U.S. Pat. No. 7,052,263 for an example of a system that utilizes this type of imaging source. In some cases, a second illumination is beneficial to improve image resolution; please see, for example, European Patent EP1744871B1.
Secondly, the recoating process and optional dwell time hinder the speed of a stereolithography process. Because a large amount of liquid radiation curable resin is present in the vat, the recoating process and dwell time cannot be entirely removed with current stereolithography processes and liquid radiation curable resin technology. The speed of the recoating process and dwell time is largely a function of the properties of the liquid radiation curable resin, primarily the viscosity.
Systems have been developed wherein the imaging occurs using a vat-based system that is modified to use a foil or film to aid in forming each layer. Such a technique is disclosed in, for example, U.S. Pat. Nos. 5,171,490, 7,052,263, and 7,438,846.
Further additive fabrication systems have been developed that are known as vat-less systems. Vat-less systems differ from traditional vat-based systems in that the imaging step does not occur within the reservoir of liquid radiation curable resin. Instead, the layer of liquid radiation curable resin is transported to the imaging area one layer at a time. Examples of vat-less systems are disclosed in, for example, European Patent EP1710625 and U.S. Pat. Nos. 6,547,552, 7,614,866, 7,758,799, and 7,731,887. An example of a commercially available vat-less system is the V-FLASH® system available from 3D Systems, Inc.
One common feature of these examples is that they require a step of separating the just cured solid layer from a separating layer or carrier such as a film, foil, glass, or plate. Such separating layers and carriers will be collectively referred to as substrates throughout this application for patent. Moreover, each of these machines employs an upside down build platform wherein the part is translated vertically upwards as it is built rather than vertically downwards as in a traditional stereolithography apparatus.
Although the use of substrates in an additive fabrication process provides several improvements over traditional vat-based systems, the use of substrates also presents several challenges. For instance, the substrate-based process adds the complexity of accurately coating the substrate with liquid radiation curable resin. Furthermore, the increased speed of the process requires that proper green strength is developed in order to facilitate proper peeling from the substrate and bonding to the previously cured layer. Lastly, adhesion of the liquid radiation curable resin to the substrate must be dealt with.
Several patent applications discuss resin formulations useful in a substrate-based additive fabrication process. WO2010/027931 to 3D Systems, Inc discloses a liquid radiation curable resin that comprises only free-radically polymerizable compounds. The compositions of WO2010/027931 include blends of (meth)acrylates and urethane (meth)acrylates. U.S. Pat. No. 7,358,283, assigned to 3D Systems Inc., discloses all acrylate liquid radiation curable resins that are allegedly easily released from substrates. These compositions also require a blend of (meth)acrylates and urethane (meth)acrylates
It is well known in the field of liquid radiation curable resins that hybrid liquid radiation curable resins produce cured three-dimensional articles with the most desirable combination of mechanical properties. A hybrid liquid radiation curable resin is a liquid radiation curable resin that comprises both free radical and cationic polymerizable components and photoinitiators. It is also well known that the cationically polymerizable components of a liquid radiation curable resin primarily contribute to the desirable combination of mechanical properties in a cured three-dimensional article, however, the cationically polymerizable components of a liquid radiation curable resin polymerize at a much slower rate than the free-radically polymerizable components. Consequently, the mechanical properties of the cured three-dimensional article develop over time after the initial cure of the hybrid liquid radiation curable resin. The added complexities of known substrate-based additive fabrication processes also contribute to the difficulty of formulating a liquid radiation curable resin for substrate-based additive fabrication processes.
It would thus be desirable to develop hybrid liquid radiation curable resins that are capable of forming cured three-dimensional articles that possess excellent mechanical properties when cured in a substrate-based additive fabrication process.